1. Field of the Invention
The invention relates to a method for producing concrete roof tiles in an extrusion process and the roof tiles produced thereby. The invention also relates to an apparatus for carrying out this process. Finally, the invention relates to the concrete roof tiles obtained by the new process. The new process is in the same manner suitable for producing both plain concrete roof tiles and profiled concrete roof tiles. The invention is described below, particularly with reference to the production of profiled concrete roof tiles, although there is no intention to restrict the invention because of this.
2. Prior Art
U.S. Pat. No. 3,776,075 issued to Douglas Frederick Matthews on Dec. 4, 1973 for MANUFACTURE OF TILES discloses a process and apparatus of this general type. The content of Matthews U.S. Pat. No. 3,776,075 is incorporated herein by reference, where necessary. This known apparatus for cutting a continuously moving ribbon of plastically deformable material into individual concrete tiles allows throughput rates of 150 tiles per minute to be achieved. However, the high cutting speed required results in irregularities on the cut surfaces.
The trimming tool of the known apparatus is a trimming knife which trims the leading edge of the next molding of the moldings just cut off from the layer of fresh concrete. In this trimming operation, a short length is removed from the front edge of each molding, so that the finished moldings adjacent to one another do not stick together during subsequent curing. The strip of material cut off during trimming is removed. It is customary, at the same time, to arrange for the trimming knife to be inclined rearwardly relative to the moldings, so that the front edges of the concrete tiles have a more pleasing appearance when laid on a roof.
The prior process and apparatus presents certain disadvantages and drawbacks. For example, the concrete roof tiles obtainable by means of this known proposal have both on the rear edge and on the front edge a plane cut surface which typically possesses pores and other irregularities. The roughness and porosity of these cut surfaces are substantially greater than those of the compacted top side, with the result that the end faces of concrete roof tiles do not have the outstanding quality of the compacted top side. The quality of the rear end face of a roof tile is less important, because, on the finished roof, this rear edge is covered by the front portion of the roof tile laid above and is thus shielded against the effects of the weather. In contrast to this, on the finished roof each front edge of the concrete roof tiles is exposed to the effects of the weather, and as a result of the action of moisture which can be further intensified by frost, excessive leaching of calcium salts and the known efflorescence often occurs here. Black spots attributable to the growth of black micro-organisms, such as mold fungi, lichens, algae and the like, which feed on calcium also frequently arise.